Queryable.Select<TSource, TResult> Method (IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>)

Projects each element of a sequence into a new form by incorporating the element's index.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IQueryable<TResult> Select<TSource, TResult>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, int, TResult>> selector
)

Type Parameters

TSource

The type of the elements of source.

TResult

The type of the value returned by the function represented by selector.

Parameters

source
Type: System.Linq.IQueryable<TSource>

A sequence of values to project.

selector
Type: System.Linq.Expressions.Expression<Func<TSource, Int32, TResult>>

A projection function to apply to each element.

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> whose elements are the result of invoking a projection function on each element of source.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source or selector is null.

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) method generates a MethodCallExpression that represents calling Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) depend on the implementation of the type of the source parameter. The expected behavior is that it invokes selector on each element of source to project it into a different form.

The following code example demonstrates how to use Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) to project over a sequence of values and use the index of each element in the projected form.

            string[] fruits = { "apple", "banana", "mango", "orange", 
                                  "passionfruit", "grape" };

            // Project an anonymous type that contains the 
            // index of the string in the source array, and 
            // a string that contains the same number of characters 
            // as the string's index in the source array. 
            var query =
                fruits.AsQueryable()
                .Select((fruit, index) =>
                            new { index, str = fruit.Substring(0, index) });

            foreach (var obj in query)
                Console.WriteLine("{0}", obj);

            /*
                This code produces the following output:

                { index = 0, str =  }
                { index = 1, str = b }
                { index = 2, str = ma }
                { index = 3, str = ora }
                { index = 4, str = pass }
                { index = 5, str = grape }
            */

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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